H-Alpha Imaging of Early-Type (Sa-Sab) Spiral Galaxies I
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Hα Imaging of Early-Type(Sa-Sab) Spiral Galaxies I1 Salman Hameed2, Nick Devereux2 Astronomy Department, New Mexico State University, Las Cruces, NM 88003 ABSTRACT Hα and continuum images are presented for 27 nearby early-type(Sa-Sab) spiral galaxies. Contrary to popular perception, the images reveal copious massive star formation in some of these galaxies. A determination of the Hα morphology and a measure of the Hα luminosity suggests that early-type spirals can be classified into two broad categories based on the luminosity of largest H II region in the disk. The first category includes galaxies for which the 39 −1 individual H II regions have LHα < 10 ergs . Most of the category 1 galaxies appear to be morphologically undisturbed, but show a wide diversity in nuclear Hα properties. The second category includes galaxies which have at least one 39 −1 H II region in the disk with LHα ≥ 10 ergs . All category 2 galaxies show either prominent dust lanes or other morphological peculiarities such as tidal tails which suggests that the anomalously luminous H II regions in category 2 galaxies may have formed as a result of a recent interaction. The observations, which are part of an on-going Hα survey, reveal early-type spirals to be a heterogeneous class of galaxies that are evolving in the current epoch. We have also identified some systematic differences between the classifications of spiral galaxies in the Second General Catalog (RC2) and the Revised Shapley- Ames Catalog (RSA) which may be traced to subtle variations in the application of the criteria used for classifying spiral galaxies. An examination of earlier arXiv:astro-ph/9904402v1 29 Apr 1999 studies suggests that perceptions concerning the Hubble type dependence of star formation rates among spiral galaxies depends on the choice of catalog. Subject headings: galaxies: interactions — galaxies: spiral — HII regions — stars: formation 1Based on observations obtained with the 1.5-meter telescope at CTIO and the 3.5-meter telescope at Apache Point Observatory (APO). The APO 3.5m telescope is owned and operated by the Astrophysical Research Consortium. 2Visiting Astronomer, Cerro Tololo Inter-American Observatory. CTIO is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. –2– 1. Introduction An outstanding problem in extra-galactic astronomy is understanding the parameters that determine the structure and evolution of galaxies. A first step towards understanding the physical properties of galaxies is classification based on morphology. Spiral galaxies were first classified by Hubble according to the size of the bulge, the tightness with which the spiral arms are wound, and the resolution of individual H II regions (Hubble 1936; de Vaucouleurs 1959). Several observational studies have elucidated differences among galaxies along the Hubble sequence (for a review see Roberts & Haynes 1994; Kennicutt 1998). Observations plus modeling of broad band colors of galaxies reveal that the bulge dominated Sa galaxies are red compared to the disk dominated Sc galaxies, suggesting an older population (Larson & Tinsley 1974). Similarly, Roberts(1969) measured the atomic gas content of 75 spiral galaxies and found that the ratio, M(HI)/L(B), decreases systematically from the late-type spirals to the early-type Sa galaxies, suggesting early types to be deficient in hydrogen gas, compared to the late types. The star formation history of galaxies can be traced using the integrated Hα equivalent widths, where the Hα emission line flux is normalized by the past star formation rate through the red continuum. Kennicutt & Kent (1983) measured the Hα equivalent widths for ∼ 200 spiral galaxies and demonstrated that the Hα equivalent widths decrease systematically from late-type spirals to early-type spiral galaxies, suggesting early-types to be deficient in massive young stars compared to the late types. There are other observational results, however, which suggest that early-type spirals are not so quiescent. A recent Hubble Space Telescope(HST) study of the bulges of 75 spiral galaxies (Carollo et al. 1998) reveals a wide variety of activity, including star formation, hidden underneath the bulges of early-type spirals. Young & Knezek (1989) have showed that the dominant phase of the interstellar medium in Sa-Sab types is molecular, not atomic and that the molecular fraction is much higher in the early-types compared to the later types. The result, however, has been recently challenged by Casoli et al. (1998), who find that molecular gas comprises only about one third to one fourth of the total gas content of spirals of types Sa through Sc. A recent analysis of the Infrared Astronomical Satellite (IRAS) database by Devereux & Hameed (1997) suggests that the global massive star formation rates, as determined by 60 micron luminosity functions, are comparable in early and late-type spirals. Similarly, far infrared to blue luminosity ratios of a large sample of nearby spiral galaxies do not show any morphological dependence (Tomita et al. 1996; Devereux & Hameed 1997). Evidently, the IRAS data has revealed a previously unsuspected population of early-type spirals with –3– high massive star formation rates. The IRAS results, do not support previous claims, based on Hα equivalent widths, that massive star formation rates increase along the Hubble sequence from Sa to Sc(Kennicutt 1983; Kennicutt et al. 1994). Part of the problem is that the sample of early-type spirals selected by Kennicutt (1983) and Kennicutt (1994) is small in number and is biased towards galaxies with low values of L(FIR)/L(B) (Devereux & Hameed 1997; Usui et al. 1998). We are, therefore, conducting an Hα imaging survey of all known nearby early-type spiral galaxies in order to better understand the differences between the IRAS results and those of existing Hα studies. High resolution Hα images of nearby galaxies provide important information about the morphology and luminosity of the ionized hydrogen gas. Surprisingly few Hα images of early-type spirals exist in the published literature. The general notion that early-type spirals do not have significant massive star formation is, at least partially, responsible for the dearth of Hα observations (Young et al. 1996). The continuum morphology of early-type spirals is dominated by an ’inert’ stellar bulge which can hide star forming complexes that lie underneath them. CCD imaging allows the hidden H II regions to be revealed by subtracting the overwhelming continuum light. Despite the dearth of Hα images, our appreciation of the heterogeneous nature of early-type spirals has evolved considerably in the past quarter century. Van den Bergh(1976) and Kormendy(1977) found no H II regions in NGC 4594 and NGC 2841, leading them to speculate that the IMF in early-type spirals may be biased against the formation of massive stars. Later studies, however, showed that H II regions are indeed present in these particular galaxies (Schweizer 1978; Hodge & Kennicutt 1983; Kennicutt 1988). A detailed study of H II regions in the disks of seven Sa galaxies by Caldwell et al. (1991) found that H II regions are quite abundant in the disks but are significantly smaller than those in late-type spirals. Specifically, Caldwell et al. found that there are no H II regions in the disks of early-type spirals with luminosities > 1039ergs−1. The purpose of the present paper is to report that H II regions are not only abundant in early-type spirals but some contain giant H II regions that are comparable in size and luminosity to giant H II regions seen in late-type spirals. Our findings support recent Hα observations by Young et al. (1996) and Usui et al. (1998), who have also identified numerous early-type spirals with star formation rates comparable to the most prolifically star forming late-type spirals. In order to quantify the diverse star forming capabilities of early-type spirals, we are conducting a systematic program of Hα imaging. The results are presented here for twenty –4– seven galaxies imaged to date. The sample is described in section 2 and the observations are described in section 3. The results are presented in section 4, followed by discussion in section 5. 2. The Sample An Hα imaging survey is being conducted to investigate the star forming capabilities of nearby early-type (Sa-Sab) spiral galaxies. The target galaxies have been selected from the Nearby Galaxy Catalog (NBG)(Tully 1988) which is the largest complete compilation of bright galaxies with velocity less than 3000km/s, corresponding to a distance of 40 −1 Mpc (H0 = 75kms /Mpc). The target galaxies are listed in Table 1 with some useful observables. Distances in the NBG catalog are based on a Virgo-centric in-fall model (Tully 1988). The NBG catalog also lists morphological types for each galaxy The complete sample includes all (57) bright, m(B) ≤ 12.1 magnitude, non-interacting early-type (Sa-Sab) spirals known within 40 Mpc. For the purposes of the present work, “interacting” galaxies are defined as those which have cataloged companions within 6′of each other. The goal of the survey is to image the complete sample of nearby early-type spirals. Imaging the largest complete sample will circumvent incompleteness corrections and minimize statistical errors in quantifying the incidence of nuclear starbursts, nuclear emission line spirals, nuclear point sources, and other morphological peculiarities in nearby early-type spirals. So far we have imaged twenty-one galaxies, the results of which are presented in this paper. As part of a complimentary study we are also obtaining Hα images of twenty-one far-infrared luminous early-type spirals, identified by Devereux and Hameed (1997). Fifteen of these galaxies have m(B) ≤ 12.1 and, hence, are already included in the complete sample described above.